目录 一、环境准备50道题目练习1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数2、查询学生选课存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null)
建表共4张表,分别对应学生信息(Student)、课程信息(Course)、教师信息(Teacher)以及成绩信息(SC)
-- 学生表create table Student(sid varchar(10),sname varchar(10),sage datetime,ssex nvarchar(10));insert into Student values('01' , '赵雷' , '1990-01-01' , '男');insert into Student values('02' , '钱电' , '1990-12-21' , '男');insert into Student values('03' , '孙风' , '1990-05-20' , '男');insert into Student values('04' , '李云' , '1990-08-06' , '男');insert into Student values('05' , '周梅' , '1991-12-01' , '女');insert into Student values('06' , '吴兰' , '1992-03-01' , '女');insert into Student values('07' , '郑竹' , '1989-07-01' , '女');insert into Student values('08' , '王菊' , '1990-01-20' , '女');-- 课程表create table Course(cid varchar(10),cname varchar(10),tid varchar(10));insert into Course values('01' , '语文' , '02');insert into Course values('02' , '数学' , '01');insert into Course values('03' , '英语' , '03');-- 教师表create table Teacher(tid varchar(10),tname varchar(10));insert into Teacher values('01' , '张三');insert into Teacher values('02' , '李四');insert into Teacher values('03' , '王五');-- 成绩表create table SC(sid varchar(10),cid varchar(10),score decimal(18,1));insert into SC values('01' , '01' , 80);insert into SC values('01' , '02' , 90);insert into SC values('01' , '03' , 99);insert into SC values('02' , '01' , 70);insert into SC values('02' , '02' , 60);insert into SC values('02' , '03' , 80);insert into SC values('03' , '01' , 80);insert into SC values('03' , '02' , 80);insert into SC values('03' , '03' , 80);insert into SC values('04' , '01' , 50);insert into SC values('04' , '02' , 30);insert into SC values('04' , '03' , 20);insert into SC values('05' , '01' , 76);insert into SC values('05' , '02' , 87);insert into SC values('06' , '01' , 31);insert into SC values('06' , '03' , 34);insert into SC values('07' , '02' , 89);insert into SC values('07' , '03' , 98);
SELECT student.*,t3.sid FROM (SELECT t1.sid,t1.score FROM (SELECT sid,score FROM sc WHERE cid = "01") as t1 JOIN (SELECT sid,score FROM sc WHERE cid = "02") as t2ON t1.sid = t2.sid WHERE t1.score > t2.score) as t3 JOIN studentON t3.sid = student.sid;
结果:
+-----+-------+---------------------+------+-----+| sid | sname | sage | ssex | sid |+-----+-------+---------------------+------+-----+| 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 02 || 04 | 李云 | 1990-08-06 00:00:00 | 男 | 04 |+-----+-------+---------------------+------+-----+2 rows in set
解析:
先将课程为01和02的课程及对应分数筛选出来,再join,on为01.sid = 02.sid,条件为01.score >02.score,结果’存’为新表t3,再将Student表和t3表join
SELECT * FROM (SELECT * FROM sc WHERE cid = "01") as t1 LEFT JOIN (SELECT * FROM sc WHERE cid = "02") as t2ON t1.sid = t2.sid;
结果:
+-----+-----+-------+------+------+-------+| sid | cid | score | sid | cid | score |+-----+-----+-------+------+------+-------+| 01 | 01 | 80.0 | 01 | 02 | 90.0 || 02 | 01 | 70.0 | 02 | 02 | 60.0 || 03 | 01 | 80.0 | 03 | 02 | 80.0 || 04 | 01 | 50.0 | 04 | 02 | 30.0 || 05 | 01 | 76.0 | 05 | 02 | 87.0 || 06 | 01 | 31.0 | NULL | NULL | NULL |+-----+-----+-------+------+------+-------+6 rows in set
解析:
即找出学生选了01课程没有选02课程的情况,用left join即可
#多表联合查询SELECT sc.sid,student.sname,avg(sc.score) FROM sc ,student WHERE sc.sid = student.sid GROUP BY sc.sid HAVING avg(sc.score) > 60;#多表连接查询SELECT sc.sid,student.sname,avg(sc.score) FROM sc JOIN student on sc.sid = student.sid GROUP BY sc.sid HAVING avg(sc.score) > 60;
结果:
+-----+-------+---------------+| sid | sname | avg(sc.score) |+-----+-------+---------------+| 01 | 赵雷 | 89.66667 || 02 | 钱电 | 70.00000 || 03 | 孙风 | 80.00000 || 05 | 周梅 | 81.50000 || 07 | 郑竹 | 93.50000 |+-----+-------+---------------+5 rows in set
解析:
首先确定的是两张表,student和sc,这里使用多表联合查询和多表连接查的方式都可以,关联条件是sid,然后分组,最后加一个having函数,条件是平均成绩大于60,即可查询出来
#多表联合查询方式SELECT t1.*,t2.score FROM student t1, sc t2 WHERE t1.sid = t2.sid GROUP BY t1.sid;#多表连接查询方式SELECT a.*,b.score FROM student as a JOIN sc AS b ON a.sid = b.sid GROUP BY a.sid;
结果:
+-----+-------+---------------------+------+-------+| sid | sname | sage | ssex | score |+-----+-------+---------------------+------+-------+| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 80.0 || 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 70.0 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 80.0 || 04 | 李云 | 1990-08-06 00:00:00 | 男 | 50.0 || 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 76.0 || 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 31.0 || 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 89.0 |+-----+-------+---------------------+------+-------+7 rows in set
解析:
确定是两个表,student和sc,关联条件还是sid消除笛卡尔积,然后再group by,最后select 取需要的信息
#多表联合查询方式SELECT t1.sid as 学生编号,t1.sname as 学生姓名,COUNT(t2.cid) as 选课总数,SUM(t2.score) as 课程成绩总和 FROM student t1, sc t2 WHERE t1.sid = t2.sid GROUP BY t1.sid;#多表连接查询SELECT t1.sid as 学生编号,t1.sname as 学生姓名,COUNT(t2.cid) as 选课总数,SUM(t2.score) as 课程成绩总和 FROM student t1 JOIN sc t2 ON t1.sid = t2.sid GROUP BY t1.sid;
结果:
+----------+----------+----------+--------------+| 学生编号 | 学生姓名 | 选课总数 | 课程成绩总和 |+----------+----------+----------+--------------+| 01 | 赵雷 | 3 | 269.0 || 02 | 钱电 | 3 | 210.0 || 03 | 孙风 | 3 | 240.0 || 04 | 李云 | 3 | 100.0 || 05 | 周梅 | 2 | 163.0 || 06 | 吴兰 | 2 | 65.0 || 07 | 郑竹 | 2 | 187.0 |+----------+----------+----------+--------------+7 rows in set
解析:
两个聚合函数(统计函数)一个count(cid),一个sum(score),同样join student表和sc表,再group by sid即可
SELECT COUNT(t.tid) FROM teacher t WHERE t.tname like "%李%";
结果:
+--------------+| COUNT(t.tid) |+--------------+| 1 |+--------------+1 row in set
解析:
count加条件函数加通配符即可
SELECT s.*,y.tname FROM (SELECT sc.sid,x.tname FROM (SELECT t.tname,c.cid FROM teacher AS t JOIN course AS c ON t.tid = c.tid WHERE t.tname = '张三') AS x JOIN sc ON x.cid = sc.cid) AS yJOIN student AS sON y.sid = s.sid;
结果:
+-----+-------+---------------------+------+-------+| sid | sname | sage | ssex | tname |+-----+-------+---------------------+------+-------+| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 张三 || 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 张三 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 张三 || 04 | 李云 | 1990-08-06 00:00:00 | 男 | 张三 || 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 张三 || 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 张三 |+-----+-------+---------------------+------+-------+6 rows in set
解析:
四表连接,teacher表里的tid与course表里的tid,条件为tname=‘张三’,再course表里的cid与sc表里的cid,最后sc表里的sid与student里的sid
SELECT a.*,count(b.cid) AS 所学课程数FROM student AS a LEFT JOIN sc AS b ON a.sid = b.sid GROUP BY a.sid HAVING COUNT(b.cid) < (SELECT COUNT(c.cid) FROM course as c);
结果:
+-----+-------+---------------------+------+------------+| sid | sname | sage | ssex | 所学课程数 |+-----+-------+---------------------+------+------------+| 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 2 || 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 2 || 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 2 || 08 | 王菊 | 1990-01-20 00:00:00 | 女 | 0 |+-----+-------+---------------------+------+------------+
解析:
先查询总课程数,再查询所有同学的信息,筛选条件为其所学课程数小于总课程数
SELECT s.* FROM student AS s JOIN sc ON s.sid = sc.sid WHERE sc.cid in (SELECT sc.cid FROM sc AS sc WHERE sc.sid = '01') GROUP bY s.sid HAVING s.sid != '01';
结果:
+-----+-------+---------------------+------+| sid | sname | sage | ssex |+-----+-------+---------------------+------+| 02 | 钱电 | 1990-12-21 00:00:00 | 男 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 || 04 | 李云 | 1990-08-06 00:00:00 | 男 || 05 | 周梅 | 1991-12-01 00:00:00 | 女 || 06 | 吴兰 | 1992-03-01 00:00:00 | 女 || 07 | 郑竹 | 1989-07-01 00:00:00 | 女 |+-----+-------+---------------------+------+6 rows in set
解析:
先从成绩表里查询学号为01的同学所学的课程编号,筛选条件为sc.cid in 01同学所学编号,再使用学生表和成绩表两表关联,关联字段为sid,并且把课程编号作为子查询的条件,刷选,然后再group by sid 最后通过having筛选sid 不等于01
select * from student t1 where t1.sid not in (select p.sid fro (select t.*,sc.sid sc_sid ,sc.cid sc_cid from (select * from student , (select cid from sc where sid = "01") s ) t left join sc on t.sid = sc.sid and t.cid = sc.cid) p where sc_sid is null) and t1.sid != "01"and (SELECT count(t3.cid) from sc t3 where t3.sid = t1.sid) = (SELECT COUNT(*) FROM sc t2 WHERE t2.sid = "01");
结果:
+-----+-------+---------------------+------+---------------+| sid | sname | sage | ssex | count(t3.cid) |+-----+-------+---------------------+------+---------------+| 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 3 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 3 || 04 | 李云 | 1990-08-06 00:00:00 | 男 | 3 |+-----+-------+---------------------+------+---------------+3 rows in set
解析:
先从成绩表中查询学号为01的总课程数,然后使用学生表和成绩表关联查询,关联字段为sid,消除笛卡尔积,where条件语句过滤学号01,并且用学号字段分组,并且使用having函数,统计课程总数=学号为1的课程总数
#多表联合查询方式SELECT student.sname FROM student WHERE student.sid NOT IN (SELECT sc.sid FROM sc JOIN course ON sc.cid=course.cid JOIN teacher ON course.tid=teacher.tid WHERE tname='张三' );#多表连接查询方式SELECT student.sname FROM student WHERE student.sid NOT IN (SELECT sc.sid FROM sc,course,teacher where sc.cid = course.cid and course.tid=teacher.tid and tname='张三');
结果:
+-------+| sname |+-------+| 吴兰 || 王菊 |+-------+2 rows in set
解析:
先找出所有学生选课信息及sid,再找出张三老师授课课程,将其连接,再用student里的sid not in 前面的sid
SELECT c.sname, b.*FROM student c JOIN (( SELECT sid, COUNT(cid) FROM sc WHERE score < 60 GROUP BY sid HAVING COUNT(cid) >= 2 ) a JOIN ( SELECT sid, avg(score) FROM sc GROUP BY sid ) b ON a.sid = b.sid) ON c.sid = b.sid;
结果:
+-------+-----+------------+| sname | sid | avg(score) |+-------+-----+------------+| 李云 | 04 | 33.33333 || 吴兰 | 06 | 32.50000 |+-------+-----+------------+2 rows in set
解析:
先查询出不及格两门或两门以上的数据,再查询出不及格的平均成绩,再三张表嵌套关联
#多表联合查询方式SELECT b.*, a.scoreFROM student b JOIN ( SELECT * FROM sc WHERE cid = '01' AND score < 60 ORDER BY score DESC ) a ON a.sid = b.sid;#多表连接查询方式SELECT b.*, a.scoreFROM student b, ( SELECT * FROM sc WHERE cid = '01' AND score < 60 ORDER BY score DESC ) a where a.sid = b.sid;
结果:
+-----+-------+---------------------+------+-------+| sid | sname | sage | ssex | score |+-----+-------+---------------------+------+-------+| 04 | 李云 | 1990-08-06 00:00:00 | 男 | 50.0 || 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 31.0 |+-----+-------+---------------------+------+-------+2 rows in set
解析:
先查询出01课程分数小于60的sid ,按照分数降序,然后和学生表关联
SELECT a.sid, a.score, a.cid, b.`平均成绩`FROM sc a JOIN ( SELECT sid, avg(score) AS 平均成绩 FROM sc GROUP BY sid ) b ON a.sid = b.sidORDER BY b.`平均成绩` DESC;
结果:
+-----+-------+-----+----------+| sid | score | cid | 平均成绩 |+-----+-------+-----+----------+| 07 | 89.0 | 02 | 93.50000 || 07 | 98.0 | 03 | 93.50000 || 01 | 80.0 | 01 | 89.66667 || 01 | 90.0 | 02 | 89.66667 || 01 | 99.0 | 03 | 89.66667 || 05 | 76.0 | 01 | 81.50000 || 05 | 87.0 | 02 | 81.50000 || 03 | 80.0 | 01 | 80.00000 || 03 | 80.0 | 02 | 80.00000 || 03 | 80.0 | 03 | 80.00000 || 02 | 70.0 | 01 | 70.00000 || 02 | 60.0 | 02 | 70.00000 || 02 | 80.0 | 03 | 70.00000 || 04 | 50.0 | 01 | 33.33333 || 04 | 30.0 | 02 | 33.33333 || 04 | 20.0 | 03 | 33.33333 || 06 | 31.0 | 01 | 32.50000 || 06 | 34.0 | 03 | 32.50000 |+-----+-------+-----+----------+18 rows in set
解析:
先求平均成绩,注意,这里的平均成绩一定要取别名,然后取所有人的成绩,再关联,然后按照平均成绩降序排列
以如下形式显示:
课程 id,最高分,最低分,平均分,及格率,中等率,优良率,优秀率
及格为>=60,中等为:[70,80),优良为:[80-90),优秀为:>=90
要求输出课程号和选修人数,查询结果按人数降序排列,若人数相同,按课程号升序
SELECT cid AS 课程id, MAX(score) AS 最高分, MIN(score) AS 最低分, AVG(score) AS 平均分 , SUM(CASE WHEN score >= 60 THEN 1 ELSE 0 END) / COUNT(sid) AS 及格率 , SUM(CASE WHEN score >= 70 AND score < 80 THEN 1 ELSE 0 END) / count(sid) AS 中等率 , SUM(CASE WHEN score >= 80 AND score < 90 THEN 1 ELSE 0 END) / count(sid) AS 优良率 , SUM(CASE WHEN score >= 90 THEN 1 ELSE 0 END) / count(sid) AS 优秀率FROM scGROUP BY cidORDER BY cid ASC;
结果:
+--------+--------+--------+----------+--------+--------+--------+--------+| 课程id | 最高分 | 最低分 | 平均分 | 及格率 | 中等率 | 优良率 | 优秀率 |+--------+--------+--------+----------+--------+--------+--------+--------+| 01 | 80.0 | 31.0 | 64.50000 | 0.6667 | 0.3333 | 0.3333 | 0.0000 || 02 | 90.0 | 30.0 | 72.66667 | 0.8333 | 0.0000 | 0.5000 | 0.1667 || 03 | 99.0 | 20.0 | 68.50000 | 0.6667 | 0.0000 | 0.3333 | 0.3333 |+--------+--------+--------+----------+--------+--------+--------+--------+3 rows in set
解析:
重点在case when语句的用法,其实case when 就类似于 if函数 if x>某个值,then 1 else 0。就只用一个表,只是对表头需要做修改,用聚合函数+AS
-- Mysql8.0以上select *, rank() over(partition by cid order by score desc) AS ranked from sc;-- mysql5.7为实现分组组内排名select s.*, @rank:=@rank+1 as ranked from sc as s,(SELECT @rank:=0) as p ORDER BY score desc;
结果:
+-----+-----+-------+--------+| sid | cid | score | ranked |+-----+-----+-------+--------+| 01 | 01 | 80.0 | 1 || 03 | 01 | 80.0 | 1 || 05 | 01 | 76.0 | 3 || 02 | 01 | 70.0 | 4 || 04 | 01 | 50.0 | 5 || 06 | 01 | 31.0 | 6 || 01 | 02 | 90.0 | 1 || 07 | 02 | 89.0 | 2 || 05 | 02 | 87.0 | 3 || 03 | 02 | 80.0 | 4 || 02 | 02 | 60.0 | 5 || 04 | 02 | 30.0 | 6 || 01 | 03 | 99.0 | 1 || 07 | 03 | 98.0 | 2 || 02 | 03 | 80.0 | 3 || 03 | 03 | 80.0 | 3 || 06 | 03 | 34.0 | 5 || 04 | 03 | 20.0 | 6 |+-----+-----+-------+--------+18 rows in set
解析:
注意:mysql8.0之前 是没有rank函数
MySQL可以实现oracle中的排名公式,一共有三种
此题目要按照各科成绩进行排序 over()中要填partition by col_name order by col_name
第一个colname 为分组的内容,第二个是按什么值排的内容
-- MySql8.0以上SELECT a.*, rank() OVER (ORDER BY a.总成绩 DESC) AS RankedFROM ( SELECT *, SUM(score) AS 总成绩 FROM sc GROUP BY sid) a;-- MySql5.7SELECT a.*, @rank := @rank + 1 AS rankedFROM ( SELECT s.*, SUM(score) AS 总成绩 FROM sc s GROUP BY sid) a, ( SELECT @rank := 0 ) pORDER BY a.总成绩 DESC;
结果:
+-----+-----+-------+--------+--------+| sid | cid | score | 总成绩 | Ranked |+-----+-----+-------+--------+--------+| 01 | 01 | 80.0 | 269.0 | 1 || 03 | 01 | 80.0 | 240.0 | 2 || 02 | 01 | 70.0 | 210.0 | 3 || 07 | 02 | 89.0 | 187.0 | 4 || 05 | 01 | 76.0 | 163.0 | 5 || 04 | 01 | 50.0 | 100.0 | 6 || 06 | 01 | 31.0 | 65.0 | 7 |+-----+-----+-------+--------+--------+7 rows in set
解析:
跟上题一样用rank()over(),只是多了层嵌套
SELECT a.*, dense_rank() OVER (ORDER BY a.total_socre DESC) AS RankedFROM ( SELECT *, SUM(score) AS total_socre FROM sc GROUP BY sid) a;
结果:
+-----+-----+-------+-------------+--------+| sid | cid | score | total_socre | Ranked |+-----+-----+-------+-------------+--------+| 01 | 01 | 80.0 | 269.0 | 1 || 03 | 01 | 80.0 | 240.0 | 2 || 02 | 01 | 70.0 | 210.0 | 3 || 07 | 02 | 89.0 | 187.0 | 4 || 05 | 01 | 76.0 | 163.0 | 5 || 04 | 01 | 50.0 | 100.0 | 6 || 06 | 01 | 31.0 | 65.0 | 7 |+-----+-----+-------+-------------+--------+7 rows in set
解析:
和上面一样,只是换成dense_rank () over(),只是总分没有重复无法看出区别
SELECT cid AS 课程ID, SUM(CASE WHEN score <= 60 THEN 1 ELSE 0 END)/count(sid) AS 百分比1,SUM(CASE WHEN score >60 AND score <=70 THEN 1 ELSE 0 END)/count(sid) AS 百分比2,SUM(CASE WHEN score >70 AND score <=85 THEN 1 ELSE 0 END)/count(sid) AS 百分比3,SUM(CASE WHEN score >85 THEN 1 ELSE 0 END)/count(sid) AS 百分比4FROM sc GROUP BY cid ORDER BY cid;
结果:
+--------+---------+---------+---------+---------+| 课程ID | 百分比1 | 百分比2 | 百分比3 | 百分比4 |+--------+---------+---------+---------+---------+| 01 | 0.3333 | 0.1667 | 0.5000 | 0.0000 || 02 | 0.3333 | 0.0000 | 0.1667 | 0.5000 || 03 | 0.3333 | 0.0000 | 0.3333 | 0.3333 |+--------+---------+---------+---------+---------+3 rows in set
解析:
使用case when
-- MySql8.0以上SELECT * FROM(SELECT *,rank() over(PARTITION by cid ORDER BY score desc) as ranked FROM sc) as aWHERE a.ranked <=3;-- MySql5.7SELECT *FROM scWHERE ( SELECT count(*) FROM sc a WHERE sc.CId = a.CId AND sc.score < a.score) < 3ORDER BY CId ASC, sc.score DESC;
结果:
+-----+-----+-------+--------+| sid | cid | score | ranked |+-----+-----+-------+--------+| 01 | 01 | 80.0 | 1 || 03 | 01 | 80.0 | 1 || 05 | 01 | 76.0 | 3 || 01 | 02 | 90.0 | 1 || 07 | 02 | 89.0 | 2 || 05 | 02 | 87.0 | 3 || 01 | 03 | 99.0 | 1 || 07 | 03 | 98.0 | 2 || 02 | 03 | 80.0 | 3 || 03 | 03 | 80.0 | 3 |+-----+-----+-------+--------+ 10 rows in set
解析:
与上面rank一样,用rank()over()where ranked <=3
注意!where 的执行顺序在select前,嵌套一个select 语句就好
MySql5.7版本:使用嵌套循环找出cid相同并且比自己score大但不超过三条的数据(前三名)
SELECT cid AS 课程id, COUNT(sid) AS 选修的学生数FROM scGROUP BY cidORDER BY 课程id;
结果:
+--------+--------------+| 课程id | 选修的学生数 |+--------+--------------+| 01 | 6 || 02 | 6 || 03 | 6 |+--------+--------------+3 rows in set
解析:
单表 查询,使用group by ,order by
SELECT student.sname, a.*FROM student JOIN ( SELECT sid, count(cid) AS 选修课程数 FROM sc GROUP BY sid HAVING 选修课程数 = 2 ) a ON student.sid = a.sid;
结果:
+-------+-----+------------+| sname | sid | 选修课程数 |+-------+-----+------------+| 周梅 | 05 | 2 || 吴兰 | 06 | 2 || 郑竹 | 07 | 2 |+-------+-----+------------+3 rows in set
解析:
先从成绩表中查询出只选修两门课程的学生id和课程数,再和学生表进行关联查询
SELECT ssex,COUNT(sid) FROM student GROUP BY ssex;
结果:
+------+------------+| ssex | COUNT(sid) |+------+------------+| 男 | 4 || 女 | 4 |+------+------------+2 rows in set
解析:
根据ssex group by后再count()
SELECT * FROM student WHERE sname like "%风%";
结果:
+-----+-------+---------------------+------+| sid | sname | sage | ssex |+-----+-------+---------------------+------+| 03 | 孙风 | 1990-05-20 00:00:00 | 男 |+-----+-------+---------------------+------+1 row in set
解析:
通配符,%,‘%a’a结尾,‘a%’a开头,‘%a%’含有a
SELECT *, COUNT(sid) AS 同名人数FROM ( SELECT a.* FROM student a JOIN student b WHERE a.sname = b.sname AND a.ssex = b.ssex) cGROUP BY sidHAVING 同名人数 >= 2;
结果:
解析:
连接表student和student on ssname and ssex 在group by sid(因为id唯一,name可能重名),count sid
SELECT * FROM student WHERE YEAR(sage) = 1990;
结果:
+-----+-------+---------------------+------+| sid | sname | sage | ssex |+-----+-------+---------------------+------+| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 || 02 | 钱电 | 1990-12-21 00:00:00 | 男 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 || 04 | 李云 | 1990-08-06 00:00:00 | 男 || 08 | 王菊 | 1990-01-20 00:00:00 | 女 |+-----+-------+---------------------+------+5 rows in set
解析:
sage一列为datetime类型,用时间函数。MySQL里面能够对datetime类型函数截取年、月、周、日等等 ,用YEAR()来表示年,以此类推
SELECT cid,avg(score) AS 平均成绩 FROM sc GROUP BY cid ORDER BY 平均成绩 DESC,cid ASC;
结果:
+-----+----------+| cid | 平均成绩 |+-----+----------+| 02 | 72.66667 || 03 | 68.50000 || 01 | 64.50000 |+-----+----------+3 rows in set
解析:
order by x desc,y,z,… 先根据x排序,再根据y,然后z…
SELECT student.sname, a.*FROM student JOIN ( SELECT sid AS 学号, avg(score) AS 平均成绩 FROM sc GROUP BY sid HAVING 平均成绩 > 85 ) a ON student.sid = a.学号;
结果:
+-------+------+----------+| sname | 学号 | 平均成绩 |+-------+------+----------+| 赵雷 | 01 | 89.66667 || 郑竹 | 07 | 93.50000 |+-------+------+----------+2 rows in set
解析:
先从成绩表中查询出平均成绩大于85的学生好和平均成绩(记住,这里需要取别名),然后再和学生表关联,关联字段为sid,获取到学生名字
SELECT student.sname, c.*FROM student JOIN ( SELECT t1.cname, t2.score, t2.sid FROM course t1 JOIN sc t2 ON t1.cid = t2.cid WHERE t2.score < 60 AND t1.cname = '数学' ) c ON student.sid = c.sid;
结果:
+-------+-------+-------+-----+| sname | cname | score | sid |+-------+-------+-------+-----+| 李云 | 数学 | 30.0 | 04 |+-------+-------+-------+-----+1 row in set
解析:
先把课程表和成绩表关联,获取到低于60分的学生号、分数和课程名称,作为临时表,然后再和学生表关联,获取到最后一个字段,学生姓名
SELECT student.sname, c.*FROM student JOIN ( SELECT a.cname, b.sid, b.score FROM course a LEFT JOIN sc b ON a.cid = b.cid ) c ON student.sid = c.sid;
结果:
+-------+-------+-----+-------+| sname | cname | sid | score |+-------+-------+-----+-------+| 赵雷 | 语文 | 01 | 80.0 || 赵雷 | 数学 | 01 | 90.0 || 赵雷 | 英语 | 01 | 99.0 || 钱电 | 语文 | 02 | 70.0 || 钱电 | 数学 | 02 | 60.0 || 钱电 | 英语 | 02 | 80.0 || 孙风 | 语文 | 03 | 80.0 || 孙风 | 数学 | 03 | 80.0 || 孙风 | 英语 | 03 | 80.0 || 李云 | 语文 | 04 | 50.0 || 李云 | 数学 | 04 | 30.0 || 李云 | 英语 | 04 | 20.0 || 周梅 | 语文 | 05 | 76.0 || 周梅 | 数学 | 05 | 87.0 || 吴兰 | 语文 | 06 | 31.0 || 吴兰 | 英语 | 06 | 34.0 || 郑竹 | 数学 | 07 | 89.0 || 郑竹 | 英语 | 07 | 98.0 |+-------+-------+-----+-------+18 rows in set
解析:
先把课程表和成绩表关联,关联字段为cid,获取到课程名称,学生号和学科成绩,作为临时表,然后再和学生表关联,关联字段为sid,获取到学生名字
SELECT student.sname, c.*FROM student JOIN ( SELECT a.cname, b.sid, b.score FROM course a LEFT JOIN sc b ON a.cid = b.cid ) c ON student.sid = c.sidWHERE c.score > 70;
结果:
+-------+-------+-----+-------+| sname | cname | sid | score |+-------+-------+-----+-------+| 赵雷 | 语文 | 01 | 80.0 || 赵雷 | 数学 | 01 | 90.0 || 赵雷 | 英语 | 01 | 99.0 || 钱电 | 英语 | 02 | 80.0 || 孙风 | 语文 | 03 | 80.0 || 孙风 | 数学 | 03 | 80.0 || 孙风 | 英语 | 03 | 80.0 || 周梅 | 语文 | 05 | 76.0 || 周梅 | 数学 | 05 | 87.0 || 郑竹 | 数学 | 07 | 89.0 || 郑竹 | 英语 | 07 | 98.0 |+-------+-------+-----+-------+11 rows in set
解析:
在上一题的基础上增加score > 70,使用where 或and都可以
SELECT cname, a.*FROM course JOIN ( SELECT score, cid FROM sc WHERE score < 60 ) a ON course.cid = a.cid;
结果
+-------+-------+-----+| cname | score | cid |+-------+-------+-----+| 语文 | 50.0 | 01 || 数学 | 30.0 | 02 || 英语 | 20.0 | 03 || 语文 | 31.0 | 01 || 英语 | 34.0 | 03 |+-------+-------+-----+5 rows in set
解析:
先从成绩表中获取到不及格的课程id和成绩,然后再和课程表关联,关联字典为课程id,获取到课程名称
SELECT student.sname,c.* FROM student JOIN(SELECT b.sid ,b.score,a.cid ,a.cname FROM course as aJOINsc as bON a.cid = b.cid WHERE a.cid = "01" AND b.score > 60) as c ON student.sid = c.sid;
结果:
+-------+-----+-------+-----+-------+| sname | sid | score | cid | cname |+-------+-----+-------+-----+-------+| 赵雷 | 01 | 80.0 | 01 | 语文 || 钱电 | 02 | 70.0 | 01 | 语文 || 孙风 | 03 | 80.0 | 01 | 语文 || 周梅 | 05 | 76.0 | 01 | 语文 |+-------+-----+-------+-----+-------+4 rows in set
解析:
先从课程表和成绩表中获取到学生号、成绩、课程号和课程名称,关联字段为课程号,作为临时表,然后再和学生表关联,关联字段为学生号,获取到学生名字
SELECT course.cname, a.*FROM course JOIN ( SELECT count(sid), cid FROM sc GROUP BY cid ) a ON course.cid = a.cid;
结果:
+-------+------------+-----+| cname | count(sid) | cid |+-------+------------+-----+| 语文 | 6 | 01 || 数学 | 6 | 02 || 英语 | 6 | 03 |+-------+------------+-----+3 rows in set
解析:
先从成绩表中统计出每门课程的人数,再和课程表关联,关联字段为课程号,获取到课程名称
SELECT student.sname, e.*FROM student JOIN ( SELECT MAX(d.score), c.*, d.sid FROM sc d JOIN ( SELECT a.tid, a.tname, b.cid, b.cname FROM teacher a JOIN course b ON a.tid = b.tid WHERE a.tname = '张三' ) c ON d.cid = c.cid ) e ON student.sid = e.sid;
结果:
+-------+--------------+-----+-------+-----+-------+-----+| sname | MAX(d.score) | tid | tname | cid | cname | sid |+-------+--------------+-----+-------+-----+-------+-----+| 赵雷 | 90.0 | 01 | 张三 | 02 | 数学 | 01 |+-------+--------------+-----+-------+-----+-------+-----+1 row in set
解析:
教师表和课程表关联,获取到教师编号、教师名称和课程编号和课程名称,关联字段为教师编号
作为临时表再和成绩表关联,关联字段为课程编号
作为临时表再和学生表关联,关联字段为学生号
SELECT student.sname, e.*FROM student JOIN ( SELECT MAX(d.score), c.*, d.sid , rank() OVER (ORDER BY MAX(d.score)) AS Ranked FROM sc d JOIN ( SELECT a.tid, a.tname, b.cid, b.cname FROM teacher a JOIN course b ON a.tid = b.tid WHERE a.tname = '张三' ) c ON d.cid = c.cid ) e ON student.sid = e.sidWHERE e.Ranked;
结果:
+-------+--------------+-----+-------+-----+-------+-----+--------+| sname | MAX(d.score) | tid | tname | cid | cname | sid | Ranked |+-------+--------------+-----+-------+-----+-------+-----+--------+| 赵雷 | 90.0 | 01 | 张三 | 02 | 数学 | 01 | 1 |+-------+--------------+-----+-------+-----+-------+-----+--------+1 row in set
解析:
用rank函数,然后再嵌套一个select,where rank = 1
SELECT DISTINCT a.*FROM sc a JOIN sc b ON a.score = b.score AND a.cid != b.cid;
结果:
+-----+-----+-------+| sid | cid | score |+-----+-----+-------+| 02 | 03 | 80.0 || 03 | 02 | 80.0 || 03 | 03 | 80.0 || 01 | 01 | 80.0 || 03 | 01 | 80.0 |+-----+-----+-------+5 rows in set
解析:
sc表自连,distinct去重,cid 不同,score相同
-- MySql8.0以上SELECT * FROM (SELECT *,dense_rank()over(PARTITION BY cid ORDER BY score DESC) AS ranked FROM sc ) aWHERE a.ranked <=2;-- MySql5.7SELECT *FROM scWHERE ( SELECT count(*) FROM sc a WHERE sc.CId = a.CId AND sc.score < a.score) < 2ORDER BY CId ASC, sc.score DESC;
结果:
+-----+-----+-------+--------+| sid | cid | score | ranked |+-----+-----+-------+--------+| 01 | 01 | 80.0 | 1 || 03 | 01 | 80.0 | 1 || 05 | 01 | 76.0 | 2 || 01 | 02 | 90.0 | 1 || 07 | 02 | 89.0 | 2 || 01 | 03 | 99.0 | 1 || 07 | 03 | 98.0 | 2 |+-----+-----+-------+--------+7 rows in set
解析:
我认为最好的前两名是排名的前2个,即第一个排名1 和第二个排名2,如果有两个并列第一,一个第二,那么前两名应该是3个人,用dense_rank,排名不跳过;如果说是最好的前两个人,就用rank,排名跳过
SELECT course.cname,a.* FROM course JOIN(SELECT cid,COUNT(sid) as 选修人数 FROM sc GROUP BY cid HAVING COUNT(sid) >5) as aON course.cid = a.cid;
结果:
+-------+-----+----------+| cname | cid | 选修人数 |+-------+-----+----------+| 语文 | 01 | 6 || 数学 | 02 | 6 || 英语 | 03 | 6 |+-------+-----+----------+3 rows in set
解析:
group by,having聚合
SELECT student.sname, a.*FROM student JOIN ( SELECT sid, COUNT(cid) AS 选修课程总数 FROM sc GROUP BY sid HAVING 选修课程总数 >= 2 ) a ON student.sid = a.sid;
结果:
+-------+-----+--------------+| sname | sid | 选修课程总数 |+-------+-----+--------------+| 赵雷 | 01 | 3 || 钱电 | 02 | 3 || 孙风 | 03 | 3 || 李云 | 04 | 3 || 周梅 | 05 | 2 || 吴兰 | 06 | 2 || 郑竹 | 07 | 2 |+-------+-----+--------------+7 rows in set
SELECT student.*, c.`选修课程总数`FROM student JOIN ( SELECT b.sid, COUNT(a.cid) AS 选修课程总数 FROM course a JOIN sc b ON a.cid = b.cid GROUP BY b.sid HAVING COUNT(a.cid) = ( SELECT COUNT(cid) FROM course ) ) c ON student.sid = c.sid;
结果:
+-----+-------+---------------------+------+--------------+| sid | sname | sage | ssex | 选修课程总数 |+-----+-------+---------------------+------+--------------+| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 3 || 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 3 || 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 3 || 04 | 李云 | 1990-08-06 00:00:00 | 男 | 3 |+-----+-------+---------------------+------+--------------+4 rows in set
解析:
从课程表中查询出总的课程数,作为后面子查询的条件
从成绩表中查询出选修了全部课程数的的学生号和选修的课程总数
作为临时表和学生表关联,关联字段为学生号,获取到全部的学生信息
SELECT sname,YEAR(NOW()) - YEAR(sage) as 年龄 FROM student;
结果:
+-------+------+| sname | 年龄 |+-------+------+| 赵雷 | 31 || 钱电 | 31 || 孙风 | 31 || 李云 | 31 || 周梅 | 30 || 吴兰 | 29 || 郑竹 | 32 || 王菊 | 31 |+-------+------+8 rows in set
解析:
使用year函数
SELECT sname , CASE WHEN DATE_FORMAT(NOW(), '%m-%d') - DATE_FORMAT(sage, '%m-%d') < 0 THEN YEAR(NOW()) - YEAR(sage) - 1 ELSE YEAR(NOW()) - YEAR(sage) END AS ageFROM student;
结果:
+-------+-----+| sname | age |+-------+-----+| 赵雷 | 31 || 钱电 | 30 || 孙风 | 31 || 李云 | 31 || 周梅 | 29 || 吴兰 | 29 || 郑竹 | 32 || 王菊 | 31 |+-------+-----+8 rows in set
解析:
有两种方法,一种是利用date_format直接截取时间类型中的月日,直接比大小
另外一种是用month()先比大小,相等再用day()比大小
SELECT sname FROM student WHERE week(NOW()) = WEEK(sage);
结果:
Empty set
解析:
week() 返回的是今年的第几周,即如果本周过生,返回数字相等
SELECT sname FROM student WHERE week(NOW()) + 1 = WEEK(sage);
结果:
Empty set
解析:
加一就行
SELECT sname FROM student WHERE month(NOW()) = month(sage);
结果:
Empty set
解析:
使用month函数
SELECT sname FROM student WHERE month(NOW()) + 1 = month(sage);
结果:
Empty set
SELECT st.*FROM student stWHERE week(now()) + 1 = week(date_format(st.s_birth, ‘ % Y % m % d’))
SELECT st.*FROM student stWHERE month(now()) = month(date_format(st.s_birth, ‘ % Y % m % d’))
SELECT st.*FROM student stWHERE month(timestampadd(month, 1, now())) = month(date_format(st.s_birth, ‘ % Y % m % d’));-- 或者SELECT st.*FROM student stWHERE (month(now()) + 1) % 12 = month(date_format(st.s_birth, ‘ % Y % m % d’));
注意:当当前月为12时,用month(now())+1为13而不是1,可用timestampadd()函数或mod取模
MYSQL窗口函数:https://blog.csdn.net/Annabel_CM/article/details/125840831
MYSQL基础常见常用语句200条:https://blog.csdn.net/c361604199/article/details/79479398
来源地址:https://blog.csdn.net/qq_42038623/article/details/128714910
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